Deferred-feature reporting

Legal Erlang that needs an unimplemented feature fails with one explicit message, distinct from ordinary invalid-input errors. Supported generic fallback (for example skipped specialization) is silent and not a failure.

[pattern matching] notimpl: src/example.erl:12:5 [module="example"] [target="native"] [operation="match argument"]

Catalog

features.hpp assigns stable, never-reused IDs and diagnostic names, with owner and status per entry. Retire or refine an entry when its feature lands; keep its ID reserved.

Owner	Where it reports
Compiler semantic analysis	Capability checks over every function, including unused code
Compiler lowering	Defensive rejection at reached operations; clears staged artifacts
Driver	Nothing deferred: `executable linking` is implemented (linking)
Runtime services	See runtime deferred services

Deferred compiler features today, each covered by codegen_placeholders:

Feature	Example that reports it
`heap expressions`	`compr_assign` comprehension assignments (`[Y \|\| X <- L, Y = X]`)
`dynamic calls`	A builtin fun of `apply` (`fun erlang:apply/2`)
`behavior-changing attributes`	`-on_load`, `-nifs`, non-inert `-compile` options, parameterized modules (accepted subset)

Message rules

The shared formatter omits unknown context, escapes control bytes as \xHH and escapes quotes and backslashes so context cannot inject lines. UTF-8 is kept.
Exactly one message per failed operation, independent of --verbose.
A deferred feature the compiler could skip without changing what it accepts would report as a warning and compilation would continue; none does today. codegen_placeholders checks one message per deferred compiler feature.
Compiler: reject_feature reports the first failure of a batch, latches failure, discards staged output and marks the diagnostic reported so drivers do not print it again.
Runtime: FeatureFailure reports once per operation through a borrowed sink (null → stderr). Later calls return the latched status. Sink or formatting failures become diagnostic_failure; exceptions are contained.
Callers propagate the failure without reporting again; a host should exit nonzero and tear down normally.